Typical data handled in computers include time-series data. The time-series data are multiple records each including time information, and the time information is referred to as a time stamp. For example, in time-series data transmitted from a temperature sensor, each record may be constituted by a time stamp and temperature information. An example of record indicates that a temperature at “10:01:27 on Jul. 14, 2009” was “27.4 degrees Celsius”. In this case, “10:01:27 on Jul. 14, 2009” represents the time stamp, and “27.4 degrees Celsius” represents main body data. There are many time-series data such as sales data of a POS terminal, position data given by a mobile unit having a GPS, load data of a computer, a log of a program, data given by an entrance/exit gate, and stock price data. The time-series data have such property that the time-series data are basically not updated except that incorrect information is corrected.
In many cases, the time-series data require processing in the order of the time stamp. However, when records of time-series data are successively received from a network, the records may not be necessarily received in the order of the time stamp. Because the order of the records may change due to a deviation of a clock at a transmitting source or due to a network delay. A time stamp of a record that could not be transmitted due to a failure and is retransmitted after recovery may be a time several days ago.
There is a limitation in the method for temporarily storing a received record to memory, sorting records in the memory, and then writing the records to a hard disk and the like in the order of the time stamp. For example, in the method described in Patent Document 1 (Japanese Laid-open patent publication NO. H09-244984), events are sorted in memory to correct the order within a range of the maximum delay time of a network.
For this reason, in general, Relational DataBase Management System (RDBMS) and the like are used to store the time-series data with indexes attached thereto. The index is meta information stored, separately from the record, in a hard disk and the like, and indicates the position in the hard disk and the like to which the record is written. When the indexes of the time stamps are generated as a B+ tree, the records can be read in the order of the time stamp without writing the records in the order of the time stamp. For example, Non-Patent Document 1 describes a system for analyzing, in an RDBMS, a large amount of time-series data generated by a communication system.